General purpose polystyrene is used in a number of the above applications. While general purpose polystyrene has a high transparency, it is brittle and has a low falling dart resistance. Additionally, it fails a crush test in which a thermoformed cup is sealed over a vacuum source. There is a mandril inside the cup to prevent complete collapse. The cup is exposed to 28" of Mercury vacuum.
The cup is observed to see if it collapses or cracks. The percent of the cups which fail or collapse and which pass is recorded.
High impact polystyrene (HIPS) is known and used in some of these applications. Cups made of HIPS pass the crush test. Unfortunately, HIPS has a low transparency, as measured on a sheet 0.040" thick, in the order of 40-50 percent.
U.S. Pat. No. 4,145,376 issued Mar. 20, 1979 to Labofina discloses a blend comprising polystyrene, a rubbery polymer such as polybutadiene and a sequenced polymer. The total amount of rubbery material in the blend is from 1 to 50 weight percent of the blend. The sequenced copolymer is used in an amount to provide from 3 to 90 weight percent of the total rubbery composition. The compositions of the present invention contain a sequenced polymer in an amount to provide over about 97 weight percent of the rubber material in the blend. The composition of the Labofina patent are opaque whereas the compositions made in accordance with the present invention have a transparency of at least 80 percent, preferably 90 percent most preferably over 95 percent.
Blends of polystyrene and radial block copolymers are known. These blends are described, for example in U.S. Pat. No. 4,467,066 issued Aug. 4, 1984 to Shell Oil Company. While these blends have a high clarity and toughness, cups made of about 50/50 blends of polystyrene and radial block copolymer do not pass the crush test. If the amount of radial block copolymer is increased the blend loses its stiffness.
Chemical Abstract 97:7267d of European Patent Application 48388 filed Mar. 12, 1982, in the name of BASF A.G., discloses a blend of 20-95 weight percent of an impact modified vinyl arene and a 1,3-diene; and 5-80 weight percent of a star-block copolymer comprising 55-80 parts of a vinyl arene and 20-45 parts of a 1,3-diene. The polymer blend has a good gloss and improved tensile strength. The exemplified blend, 90 percent impact modified styrene and 10 percent star-block copolymer would not have the high transparency required in the present invention. Furthermore, the blend of the reference contains a minimum of 20 percent of an impact copolymer of a vinyl arene and a diolefin. The impact modified copolymer, at the level at which it is used in the reference and the level of 1,3diene in impact modified copolymer of about 6 percent would contribute more particulate impact modifier than is required in the present invention. Furthermore, the abstract is silent on particle size distribution of the impact modifier.
Applicants have discovered novel thermoplastic blends which have a high resistance to impact, and good clarity. This is unexpected in view of the extremely low level of impact modifier present in the blend.